Hardwood Flooring Underlayment: What You Need, What You Don’t, and Why It Matters

Underlayment sits between the subfloor and the finished hardwood, and its job varies depending on your installation method, subfloor type, and what the floor needs to accomplish acoustically. Get the underlayment right and you have a floor that’s quiet, stable, and moisture-protected. Get it wrong and you end up with a floor that flexes under foot, squeaks, or traps moisture beneath the boards.

What We’ll Cover

• What underlayment actually does and when it’s required
• Why installation method determines underlayment type
• Every underlayment material compared: felt, foam, cork, rubber, and combination
• Acoustic underlayment for hardwood: STC, IIC, and what the numbers mean
• Underlayment for engineered hardwood flooring specifically
• How subfloor type changes the decision
• Common mistakes that lead to floor problems
• How VC Floor approaches underlayment selection
• Answers to the most common questions about hardwood flooring underlayment

What Underlayment Actually Does

Underlayment serves four distinct functions, and the priority of each depends on the specific installation.

Moisture Protection

Over concrete subfloors, moisture vapour rises continuously through the slab. Without a barrier between the slab and the flooring, that moisture reaches the underside of the hardwood boards and causes swelling, cupping, and adhesive failure over time. Underlayment with an integrated vapour barrier or a separate polyethylene sheet beneath it intercepts that moisture before it reaches the wood.

Sound Absorption

Hard floor surfaces transmit impact noise directly – footsteps, dropped objects, furniture movement – through the floor assembly into the structure and the space below. Underlayment breaks that transmission path by introducing a resilient layer that absorbs impact energy rather than passing it through. This matters in any multi-storey home but is particularly critical in condos, where acoustic ratings are often a strata or building code requirement.

Cushioning and Comfort

A resilient layer underfoot makes a floor feel softer and warmer than one laid directly over concrete. This is noticeable across a full day of standing or walking and becomes significant in kitchens, open-plan living areas, and any space where people spend extended time on their feet.

Subfloor Irregularity Compensation

Minor subfloor imperfections – small humps or low spots within tolerance – are partially absorbed by a compressible underlayment layer. This prevents the finished floor from telegraphing small irregularities from beneath and reduces board movement that leads to squeaking.

Does Hardwood Flooring Need Underlayment?

The answer depends entirely on the installation method. There is no single answer that applies across all hardwood installations.

Nail-Down Installations: Felt or Rosin Paper

For nail-down solid hardwood over a plywood or wood subfloor, a full acoustic underlayment is not required and in many cases is not appropriate. The nails anchor the boards directly to the subfloor, and a thick compressible underlayment layer would compress unevenly under load and allow the boards to move.

What is used instead is 15-pound roofing felt or rosin paper laid over the subfloor before nailing. This provides light moisture resistance, reduces wood-on-wood squeaking between the hardwood and subfloor, and gives a clean base layer for the installation. It is thin and non-compressible, so it doesn’t compromise the rigidity of the nail-down connection.

Glue-Down Installations: Moisture Barrier Only

For glue-down hardwood over concrete, the adhesive bonds the floor directly to the slab, which means a thick underlayment layer beneath the flooring is incompatible with the installation method. What is used is a moisture barrier membrane applied to the concrete surface before the adhesive, preventing moisture vapour transmission from the slab from affecting the adhesive bond and the underside of the boards.

Some adhesives on the market are formulated to serve as both adhesive and moisture barrier simultaneously, simplifying the installation.

Floating Installations: Full Underlayment Required

Floating hardwood and engineered hardwood installations – where the boards click or glue together edge-to-edge but are not fastened to the subfloor – require a proper underlayment layer beneath them. The underlayment provides the cushioning, moisture protection, and acoustic performance the floating floor needs, since it isn’t mechanically anchored to the subfloor.

This is where underlayment material and specification make the biggest practical difference, and where acoustic performance becomes a concrete decision rather than a general consideration.

Hardwood Flooring Underlayment Materials Compared

Felt Underlayment

Felt is one of the most established underlayment materials for hardwood floors and is the standard choice for nail-down installations as roofing felt or flooring felt. For floating installations, higher-density felt – typically 40 to 60 lb felt – provides meaningful sound absorption and a stable, non-compressible base. It is semi-permeable, which makes it appropriate for wood subfloors where the subfloor needs to breathe, but it doesn’t provide an integrated vapour barrier for concrete applications.

Felt is also an environmentally practical choice – most flooring felt is manufactured from recycled materials. Its acoustic performance is moderate compared to rubber and cork, but it outperforms basic foam.

Foam Underlayment

Polyethylene foam underlayment is the most widely used product for floating hardwood and engineered hardwood installations because it’s lightweight, inexpensive, fast to install, and many versions include an integrated polyethylene vapour barrier on the underside – eliminating the need for a separate moisture barrier layer over concrete.

The trade-off is durability and acoustic performance. Foam compresses over time under concentrated loads like furniture legs, which can create low spots that affect the feel of the floor above. Its IIC ratings are also lower than rubber or cork, which limits its usefulness in applications where acoustic performance must meet specific building requirements. For a residential floating floor in a single-family home without acoustic requirements, foam is a practical and cost-effective choice. For a condo with strata noise thresholds, it usually won’t be sufficient.

Cork Underlayment

Cork is a naturally resilient, thermally insulating material that absorbs sound well and resists mould and mildew growth – properties that make it well-suited for basements and below-grade installations where both moisture and acoustic performance matter. It is semi-permeable but typically used with an additional vapour barrier over concrete.

Cork is denser than foam and compresses less under load, giving it better long-term stability. It also has better thermal insulation properties than foam or felt, which is relevant in Edmonton installations where subfloor temperatures drop significantly in winter. Its acoustic performance sits between foam and rubber, making it a good mid-range option for residential floating installations where moderate sound reduction is needed without meeting the higher thresholds required in multi-unit buildings.

Rubber Underlayment

Rubber – particularly recycled rubber products – delivers the highest acoustic performance of any common underlayment material and is the standard specification for condo and multi-unit applications where STC and IIC ratings must meet building or strata requirements.

Dense recycled rubber underlayments can achieve IIC ratings of 70 to 73 and STC ratings of 60 to 66, well above the minimums required in most Vancouver and Edmonton condominium buildings. Rubber is fully waterproof, extremely durable under heavy loads without compressing permanently, and resists mould. Its higher cost compared to foam or cork is offset by its performance ceiling and longevity.

Combination Underlayment

Combination products layer multiple materials – typically foam bonded to rubber, or felt bonded to a vapour barrier film – to combine the moisture performance of one material with the acoustic performance of another. These products tend to hit a practical middle ground: better acoustic performance than pure foam, more affordable than high-specification rubber, with integrated moisture protection built in. They suit installations where acoustic performance matters but the requirements don’t reach the higher thresholds of condominium or commercial applications.

Acoustic Underlayment for Hardwood Flooring: Understanding the Ratings

For any installation in a multi-storey building – including condos, apartments, and townhouses – the acoustic specification of the underlayment is not optional. It’s typically a requirement of the building’s bylaws or strata rules.

STC and IIC: What the Numbers Mean

STC (Sound Transmission Class) measures how well a floor assembly blocks airborne sound – voices, music, and television noise passing between units. IIC (Impact Insulation Class) measures how well it absorbs impact sound – footsteps, dropped objects, and furniture movement.

Both ratings apply to the floor assembly as a whole: the subfloor, the underlayment, and the finished flooring all contribute to the combined rating. The underlayment product makes the largest single contribution to the IIC rating because it’s the layer directly responsible for absorbing impact energy before it enters the structure.

What Ratings Are Typically Required

Building code minimums in Canada set the floor for STC and IIC ratings at 50 for residential construction. In practice, many Vancouver condominium buildings – particularly post-2010 glass-and-concrete towers – require significantly higher thresholds, typically STC 65 to 72 and IIC 67 to 72. Edmonton condominium buildings typically sit in the STC 55 to 65 and IIC 55 to 65 range depending on building age.

The practical implication: for condo installations, check the building’s specific required ratings before selecting any underlayment product. A standard foam product rated IIC 52 will not meet a building requiring IIC 67. Submitting a flooring application to a strata council with an underlayment spec that doesn’t meet the minimum is one of the most reliable ways to get the application rejected.

Delta IIC vs. Absolute IIC

One detail worth understanding when comparing acoustic underlayment products is the difference between Delta IIC and absolute IIC ratings. Delta IIC measures the improvement in impact sound performance that the underlayment provides on its own, without accounting for the floor assembly. Absolute IIC measures the performance of the complete floor assembly – subfloor, underlayment, and finished flooring together.

Strata and building applications typically ask for the absolute IIC of the full assembly, not just the underlayment product’s Delta IIC. Confirm which figure a product’s spec sheet is reporting before using it in an application.

Underlayment for Engineered Hardwood Flooring Specifically

Engineered hardwood flooring underlayment choices follow the same framework as solid hardwood but with a few specifics worth noting.

Most engineered hardwood is installed as a floating floor, which means a full underlayment is appropriate and required. Some engineered products are also installed with glue-down over concrete, in which case the underlayment is replaced by a moisture barrier membrane rather than a cushioning product.

Some engineered hardwood products come with a pre-attached underlayment layer bonded to the back of each plank. Where this is the case, adding a second underlayment beneath those planks is not recommended – the combined thickness of two layers introduces too much compression under load, which affects the stability of the locking joints and can cause them to disengage over time. Check the manufacturer’s installation guidelines for the specific product before adding a separate underlayment layer.

For engineered hardwood installations in condos requiring acoustic compliance, pre-attached underlayment typically does not meet the required STC and IIC thresholds on its own. A separate, high-specification rubber or combination underlayment is necessary, which means the product selected should not have pre-attached underlayment, or the existing underlayment must be removed to make room for the compliant product.

How Subfloor Type Changes the Underlayment Decision

Over Concrete

Moisture management is the primary concern. Any product used over a concrete slab needs either an integrated vapour barrier or a separate polyethylene sheet beneath it. Rubber and closed-cell foam both work directly over concrete. Cork requires an additional vapour barrier layer. Felt without a vapour barrier is not appropriate over concrete slabs.

Over Wood or Plywood

Wood subfloors need to breathe – trapping moisture against a wood subfloor promotes mould growth and subfloor rot. Semi-permeable underlayments like felt and cork are the appropriate choices here. Fully impermeable products like rubber can be used, but leave a small gap at the perimeter or use a perforated version to allow moisture vapour to dissipate rather than accumulate.

Over Heated Subfloors

For hardwood installations over in-floor radiant heating, the underlayment must allow adequate heat transmission from the subfloor to the room above while still providing moisture protection and any required acoustic performance. Thicker or more insulating underlayments – particularly cork at high thickness – can reduce the efficiency of the radiant system significantly. For radiant heat applications, choose a product rated as compatible with in-floor heating and check the manufacturer’s recommended maximum R-value for the specific heating system.

Common Mistakes with Hardwood Flooring Underlayment

Using foam in a condo application where acoustic ratings are required. Basic foam doesn’t meet the IIC thresholds that most Vancouver and Edmonton condo buildings require. The cost of replacing a newly installed floor because the underlayment didn’t meet the building’s specs far exceeds the cost of specifying the right product from the start.

Adding underlayment to a glue-down installation. Glue-down hardwood bonds directly to the subfloor. Adding a cushioning underlayment beneath a glue-down floor introduces a layer that compresses unevenly under load, weakens the adhesive bond, and causes the floor to flex. A moisture barrier membrane is what goes under a glue-down installation, not a compressible underlayment.

Doubling up underlayment layers when the flooring has pre-attached underlayment. The combined thickness creates a floor system that’s too soft, which causes floating locking joints to flex and eventually separate.

Not testing moisture before installation over concrete. Even with an appropriate vapour barrier underlayment, installing over a concrete slab with elevated moisture content risks adhesive or moisture barrier failure. Moisture testing before laying the underlayment confirms the subfloor is within acceptable limits.

How VC Floor Approaches Underlayment Selection

VC Floor’s approach to underlayment is straightforward: the product is matched to the installation method, subfloor conditions, and the acoustic requirements of the specific building.

For condo and multi-unit installations in Vancouver and Edmonton, VC Floor selects acoustic underlayment with published STC and IIC ratings that meet the building’s specific requirements. The product spec sheets are prepared for every project and are ready to include in a condo board or strata application. For details on the installation process and how acoustic compliance is handled, visit the hardwood flooring installation service page.

For residential house installations, underlayment is selected based on the installation method, subfloor type, and whether radiant heating is involved. The same process applies to engineered hardwood – if the product has pre-attached underlayment, the specification is adjusted accordingly.

Frequently Asked Questions